Nitro-l,3-dienes

The nitro-aldol reaction followed by dehydration gives 2-nitro-l,3-dienes, which are useful reagents for cycloaddition (Eq. 3.45).70... 

At variance with selenosulfonylation, however, attempts to prepare 2-nitro-l,3-dienes by oxidative elimination of selenium from the nitroselenylated products failed, probably owing to the lability of the products, which easily undergo further transformations. The expected 2-nitro-l,3-dienes have indeed been trapped as monoepoxy derivatives. 

Enamines (e.g. 377) have been shown to react with conjugated nitroolefins 378 to give mainly dihydro-l,2-oxazine A -oxide derivatives 379 as products of kinetic control (sometimes a cyclobutane ring is formed in these reactions see Section II.B). The stability of these heterocycles is largely dependent on the parent enamine and the type of substituent used on the nitro olefin as has been extensively studied by Valentin and coworkers " . Usually they open into the corresponding nitroalkylated enamines 380 (equation 82), in particular in a solution of methanol or deuteriated chloro-and often an equilibrium between the two forms is established. Stable 1,2-oxazine A -oxides have been obtained in the reaction of 2-nitro-l,3-dienes with cyclic enamines . 

Scheme 31). The method suffers from a lack of selectivity, both stereo - and regio-isomers being formed in most cases. A variety of 1-nitro-l,3-dienes were prepared by treatment of the corresponding dienes with trifluoroacety1 nitrate (prepared in... 

Unsaturated nitro-compounds have been prepared by various methods. 1-Nitro-l,3-dienes can be prepared by elimination of... 

Interesting push-pull dienes, 1-dialkylamino-4-nitro-l,3-butadienes, are prepared by the application of this addition-elimination reaction. (Eq. 4.22).28... 

Nitro-2-phenyloxazole, obtained by thermal isomerization of the corresponding nitroi-soxazole, is found to undergo Diels-Alder reactions with 2,3-dimethylbuta-l,3-diene (see Eq. 

The authors extended their investigations to a series of functionalized dienes such as sorbic acid, methyl sorbate, 1-nitro-l,3-butadiene, and cyclic dienes. 

Quinolines have also been prepared on insoluble supports by cyclocondensation reactions and by intramolecular aromatic nucleophilic substitution (Table 15.26). Entry 10 in Table 15.26 is an example of a remarkable palladium-mediated cycloaddition of support-bound 2-iodoanilines to 1,4-dienes. Reduction of the nitro group of polystyrene-bound 2-nitro-l-(3-oxoalkyl)benzenes with SnCl2 (Entry 11, Table 15.26) leads to the formation of quinoline /Y-oxides. These intermediates can be reduced to the quinolines on solid phase by treatment with TiCl3. 4-Quinolones have been prepared by thermolysis of resin-bound 2-(arylamino)methylenemalonic esters [311]. 

However, it was recently reported that all nitrosoarenes (except the 4-nitro compound), when produced by the oxidation of arylamines P-H2NC6H4R (R = MeO, Me, Cl, COMe, CONH2, CF3) with 2.2 mol hydrogen peroxide in the presence of a catalytic amount of oxoperoxo(2,6-pyridinedicarboxylato-0,7V,<9 ) (hexamethylphosphortriamide)-molybdenum(VII) 171, react with cycohexa-l,3-diene in situ to give the bridged dihydrooxazines of equation 95 in 66-81% yields. 

The heterocyclic double bond in simple indoles will take part in cycloaddition reactions with dipolar 4n components,and with electron-deficient dienes (i.e. inverse electron demand), in most reported cases, held close nsing a tetherf° a comparable effect is seen in the intermolecular cycloaddition of 2,3-cycloalkyl-indoles to ortfto-quinone generating a l,4-dioxane. ° The introduction of electron-withdrawing substituents enhances the tendency for cycloaddition to electron-rich dienes 3-acetyl-1-phenylsulfonylin-dole, for example, undergoes aluminium-chloride-catalysed cycloaddition with isoprene, " and 3-nitro-l-phenylsulfonylindole reacts with l-acylamino-buta-l,3-dienes without the need for a catalyst. Both 3- and 2-nitro-l-phenylsulfonyl-indoles undergo dipolar cycloadditions with azomethine ylides. ... 

Observations of reactions of the present type with compounds in which the aryl groups are separated by conjugated polyenes date back to 1962.139 There have been several further reports of the process, and Leznoff and co-workers have described the reaction of 1-formylphenyl-4-phenylbuta-l,3-dienes. 140>141 The reaction does not occur with acetyl or nitro-substituents on such compounds since the lowest transition is nrr, but from l-(p-formylphenyl)-4-phenylbuta-l, 3-diene, for example, 1 -(p-formylphenyl)naphthalene and 8-phenyl-2-naphth-aldehyde are formed similar results have been reported for the /w-formylphenyl 134 ... 

Node and co-workers have found that the Diels-Alder reaction of nitroalkenes v/ith 1-methoxy-3-trimethylsilyloxy-l,3-butadiene (Danishefsky s dienesi exhibit abnormal exo-se-lecdvity Electrostadc repulsion between the nitro and the silyloxy group of the diene induces this abnormal exc-selecdvity (Tq 8 10 This selecdve reacdon has been used for the asymmetric synthesis of various naniral products as shovm in Scheme 8 6... 

This route to the a-nitroso derivatives of the 7r-deficient heterocycles has permitted an exploration of their chemistry. They are extremely reactive and condense readily with 1,3-dienes to give 3,6-dihydro-l,2-oxazines (e.g. 99), and with aromatic amines in the presence of acid to give azo compounds (Scheme 86). This latter reaction is particularly useful in view of the instability of the corresponding 2-pyridinediazonium salts referred to above, which limits conventional access. The a-nitroso heterocycles are oxidized by ozone or sodium hypochlorite to the a-nitro compounds (Scheme 86) (82JOC553). 

Ready addition of 2-nitrosopyridine 804 to 1,3-dienes gives 3,6-dihydro-l,2-oxazines, e.g., 804 803, and condensation with aromatic amines gives azo compounds, e.g., 804 805. Nitroso compounds are oxidized by ozone or sodium hypochlorite to the corresponding nitro compounds. 5-Nitrosopyrimidines can be reduced to the 5-amino derivatives or condensed with activated methylene groups. 

Dimethoxy-2-(3-nitro-1,2,4-triazol-1 -yl)benzene and 1,1,4-trimethoxy-4-(3-nitro-l,2,4-triazol-l-yl)cyclohexane-2,5-diene have been prepared by indirect electrochemical method [598],... 

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